Geoscience Reference
In-Depth Information
of individual rainstorms can become problematic (Dunkerley,
2008a
; Dunkerley,
2008b
). However, this complexity is becoming better understood and appropriate
techniques are being devised to monitor the nature and action of these processes
(Dunkerley,
2010
).
Two other processes of hill slope erosion strongly controlled by gravitational forces
are slowmass movement (soil creep and surface rock creep) and rapid mass movement
(landslides, mudflows and debris flows). There are few long-term studies of soil creep,
but what data do exist suggest that volumetric rates of soil movement by soil creep
may equal those from slopewash under the same climatic regime and on rocks of
similar type, such as sandstone or granite (Williams,
1973a
). Volumetric data on
landslide activity in desert areas are sparse, but there are many accounts of mudflows
and debris flows triggered by sudden downpours in arid and semi-arid areas. Since
Archimedes' Principle pertains to all fluids, mudflows have a far greater capacity to
transport large boulders than does an equivalent volume of less muddy water. Out
on the plains surrounding Jebel Archenu ring-complex in the south-eastern Libyan
Desert, there are large boulders up to 1 m in size that have quite probably been carried
by former debris flows, the finer portions of which have long since been washed and
blown away. A simple way to distinguish debris flow deposits from river alluvium
is to examine whether or not the clasts or rock fragments within the deposits are in
contact with each other (
clast-supported
) or are encased by finer sediment (
matrix-
supported
). Alluvial deposits are generally clast-supported. However, many alluvial
fan deposits (
Figures 10.7
and
10.8
) contain a mixture of both clast-supported and
matrix-supported materials, indicating that they were formed by both debris flows and
channel flows. Hill slopes are not the only suppliers of sediment to rivers. Another
important source comes from bed and bank erosion in gullies, arroyos and ephemeral
stream channels, to which we now turn.
10.4 Gullies, arroyos and ephemeral stream channels
In few parts of the desert world have ephemeral stream channels been investigated
more thoroughly than they have in the American Southwest (Bryan,
1925a
;Bryan,
1925b
; Leopold and Miller,
1956
; Schumm and Hadley,
1957
; Bull,
1964a
; Bull,
1964b
; Lamarche,
1966
; Leopold et al.,
1966
; Tuan,
1966
; Haynes,
1968
; Cooke
and Reeves,
1976
; Graf,
1979
; Balling and Wells, 1980; Graf,
1982
; Graf,
1983a
;
Graf,
1983b
; Graf,
1987a
; Bull,
1991
; Schumm,
1991
, pp. 108-119; Bull,
1997
;
Tucker et al.,
2006
). Three of the most comprehensive reviews of this topic are
those by Cooke and Reeves (
1976
), Graf (
1987a
)andBull(
1997
). The outstanding
investigations in the American Southwest (summarised by Graf,
1987a
) inspired a
series of comparable studies in the Negev Desert of southern Israel (Schick,
1974
;
Schick et al.,
1987
; Laronne and Reid,
1993
;Yaır and Lavee,
1976
;Yaır and Lavee,
